In many printing operations, a printer comprises multiple print units, arranged in a sequence, in a processing line. Each of the print units comprises rollers arranged and configured to impress an image on a substrate such as newsprint, as it passes between the rollers, in a well known printing technique. The rollers comprise, for example, in a offset printer, a blanket cylinder and an opposed printing plate cylinder. The print units may each apply a different color ink, for example, the three primary colors and black.
Typically, a motor is coupled to each of the cylinders of each print unit. Conventional electrical braking methods are often implemented for stopping each motor quickly, when necessary, as for example, during an emergency stop of multiple print units. The braking methods include dynamic braking. In a dynamic braking method, the flow of current in the motor armature is reversed while, at the same time, maintaining the motor field. This action effectively converts the rotating energy of the motor armature into current flow, so that the motor acts as a generator, producing a back-EMF current flow in reverse direction from the flow of drive current, a regenerative power.
A high-wattage braking resistor is then switched across the armature to dissipate this regenerated current, bringing the motor to a stop. The effective stopping speed is a function of resistance; the lower the resistor value (therefore, the greater the reversed current flowing through the armature), the faster the motor can be stopped. In many multi-print unit arrangements, all of the motors of the various print units utilize a common bus for drive current, and share a single braking resistor arrangement. The braking resistor arrangement can comprise a single resistor, or multiple resistors, such as a common arrangement of several resistors coupled in parallel to one another.
In a print unit braking operation there is an impression cylinder throw off. There can be a transient torque peak during the impression cylinder throw off. The peak condition results in a requirement for a certain size of braking resistor arrangement to handle the increased current flow caused by the surging regenerative power, during the transient event.
In an emergency stop condition, when the entire multi unit operation must be stopped, all of the motors are stopped, and all of the impression cylinders are thrown off, essentially simultaneously. This results in a cumulative transient torque peak event. Thus, the braking resistor arrangement must be of sufficient size to handle the transient event characteristics of several motors, at the same time. This requirement for a relatively large braking resistor arrangement and other corresponding braking components, adds considerable cost to the overall cost of the print unit arrangement, and also adds to the physical space requirements for the installation of the units.